In a series of related papers, M. Rakowski DuBois and co-workers have described the existence of a novel class of molybdenum-sulfide dimer compounds. See, e.g., M. Rakowski DuBois et al., J. Am. Chem. Soc., vol. 101, pp. 5245-5252 (1979); M. Rakowski Dubois et. al., Inorg. Chem., vol. 20, pp. 3064-3071 (1981); M. McKenna et al., J. Am. Chem. Soc., vol. 105, pp. 5329-5337 (1983); and J. Birnbaum et al., Organometallics, vol. 10, pp. 1779-1786 (1991). The dinuclear sulfide bridged molybdenum dimers have the general formula [(C.sub.5 H.sub.5 Mo).sub.2 (.mu.-S).sub.4-x (.mu.-SR).sub.x ].sup.n, where x=0-3, and n=0, +1, -1.
Surprisingly, it has been shown that these molybdenum-sulfide dimers will undergo reversible interactions with olefins. Of course, the reversible interactions of olefins with a variety of transition metal ions has been known for many years. For example, Ag(I) binds reversibly with olefins. However, the reactions of these dimers with olefins is fundamentally different from those characterized for silver and other metal ions because it is the sulfide ligands of the molybdenum-sulfide dimers that act as the site of olefin binding. A typical reaction is shown below. ##STR1##
A series of experiments were performed to determine the equilibrium constants for the above reaction in chloroform for a series of olefins. See, McKenna supra. The data indicate that some of the binding constants are in the same range as those observed for olefin binding to the silver ion. It was also shown that the equilibrium constants were quite sensitive to the steric and electronic features of the olefin. For example, the equilibrium constant for trans-2-butene at 26.degree. C. was found to be (3.+-.1).times.10.sup.2 while cis-2-butene was 17.+-.2 and cis-2-hexene was 3.9.+-.0.2.
In addition to its reactivity to olefins, the molybdenum-sulfide dimers of Rakowski Dubois have also been shown to form irreversible adducts with acetylene to yield thermally stable alkenedithiolate compounds as shown in a typical example below. ##STR2##
Although the alkenedithiolates formed are considerably more stable than the molybdenum-sulfide olefin adducts, they are not unreactive. See, McKenna supra. For example, exchange reactions between certain alkenedithiolates and various alkynes have been detected. It has also been shown that the alkenedithiolates can be hydrogenated under mild conditions to yield the original molybdenum-sulfide dimer and the cis-alkene as shown below: ##STR3## Under the mild reducing conditions employed, the alkanedithiolate moiety of the dimer (in the example, the methylene dithiolate) is not reduced.
In a few instances, the ability of metal ions to bind reversibly to olefins has been utilized in olefin separation and purification systems. For example, olefin adducts of Ag(I) ion have been used in chromatographic systems for the separation of olefins. More recently, aqueous silver nitrate solutions have been used to separate ethylene or propylene from purified multicomponent gas streams. See, U.S. Pat. No. 4,174,353 of Marcinkowsky et al. A major concern when utilizing the silver/olefin adduct chemistry, is that the silver ion forms a complex with acetylene which is explosive when dry and rigorous methods must be employed to remove acetylene from any gas stream that will come in contact with the silver ion. A further problem associated with silver/olefin separation schemes, is that the silver ion is rapidly poisoned by H.sub.2 S, a common impurity in gas associated with the thermal cracking of hydrocarbons.
In contrast to the use of metal ion chemistry in olefin separation schemes, the molybdenum-sulfide dimers are unaffected by the presence of H.sub.2 S. In addition, the inventors of the present invention have also described how the ability of the molybdenum-sulfide dimers to bind and to subsequently reduce alkynes may be used in alkyne removal processes.
Prior to the conception of this invention, molybdenum-sulfide dimers had not been developed for use in any way other than as an interesting and unique chemical compound. For example, the molybdenum-sulfide dimers reported to date have not been water soluble, and the olefin alkene adduct reactions have only been studied in chloroform or other organic solvents.
One of the primary features of the present invention is the separation or purification of olefin streams. Olefins are generally produced via catalytic cracking processes. Such processes produce refinery-grade olefins (65-70% purity). Currently, refinery grade olefins are further separated and purified using distillation columns to produce polymer-grade olefins (99.5% purity) or chemical-grade olefins (95% purity). Frequently two distillation columns must be employed. Each distillation step is expensive and energy intensive, and even incremental gains in purity greatly increase the costs of the olefin products.
Current theories for improving the economies of olefin separations and purifications suggest that a hybrid separation process be utilized. In addition to the conventional distillation step, some other chemically specific process would be utilized to enhance olefin purity for greatly reduced costs. Processes that have been suggested as potentially being amenable to the hybrid approach are the following: 1) a facilitated transport membrane using a chemically-specific complexing agent; 2) absorption/stripping with a chemical solvent; and 3) adsorption/desorption on a solid support.
The rationale for the use of hybrid olefin separation processes is as follows: conventional separation technology can only achieve a certain level of separation per stage. This level of separation is not a constant for each stage. As higher purity levels are required, the number of stages increases rapidly. This also means a dramatic increase in the costs for additional processing equipment. On the other hand, a separation step using reversible chemical complexation obtains improved selectivity at the same time that the driving force of conventional olefin separation processes decreases. Although this is not intuitive, it occurs because there is a large excess of complexing agent present and the selective reaction becomes very efficient. The hybrid process, therefore, typically combines a conventional separation process to achieve a certain level of purity and follows it with a separation step using reversible chemical complexation to "polish" or further purify the desired product. See, Haggin, Chem. & Eng. News., pg. 23-24 Feb. 25, 1991.
The present inventors have recognized the potential the molybdenum-sulfide dimers have as part of a hybrid olefin separation process. Previous reversible binding systems have used AgNO.sub.3 or copper compounds as complexing agents. As described above, AgNO.sub.3 reacts irreversibly with sulfur compounds. Copper compounds are very susceptible to reactions with oxygen, water or sulfur compounds. Such reactions lead to very poor lifetimes in operation or, in the alternative, lead to the higher costs required for removing the compounds prior to separation and purification. Similar problems exist with acetylene (when purifying ethylene) and propyne (when purifying propylene). Again, the alkyne impurities increase costs either by greatly reducing the lifetime of the complexing agent, or by the mechanism required to remove the alkyne prior to the separating process utilizing the complexing agent.
Acetylene removal typically may be accomplished in a variety of manners. In one process, the gaseous feed stream is passed through a chilled polar solvent such as dimethyl formamide (DMF). A pressure swing is then used to recover the acetylene. Another method is the dehydrogenation of the acetylene over a noble metal catalyst.
The present invention describes modified molybdenum-sulfide dimers having characteristics that will allow this unique family of compounds to be used in a variety of processes, including alkene separation/purification processes.